The NTMB+FL is designed to put more focus on the mid frequencies for fretless instruments.  As such, the bass control has less range than the NTMB+F ( +/-12dB @30Hz vs. +/-15dB @30Hz).  Also the treble control has less range and is set at a lower frequency than the NTMB+F ( +/-15dB @6.5KHz vs. +/-18dB @10KHz). 

This results in more treble attenuation above the control frequency on the FL model, so the voice in general is less bright.  Additionally, both the bass and treble controls of the FL model have more effect on the mid frequencies because the filter Q is lower for all 3 bands.  Overall, this makes the FL well suited for fretless instruments which tend to have less focus on both brightness and heavy bottom.